A typical harvesting machine, such as a forage harvester, may include an attachment mounted on the front end of the harvester for cutting and/or gathering crop material and feeding it into a basic processing unit. The basic unit includes feed rolls for advancing the crop material, compressing it into a mat, and feeding it into a cutterhead. The cutterhead, which generally comprises a plurality of knives mounted on a rotating reel, the knives cooperating with a stationary shear bar to chop the crop material. The crop material is then discharged from the harvester.
Tramp metal such as fencing stakes, nuts and bolts detached from other equipment, tools which have been left in the field or bounced out of moving machines, etc. is frequently picked up from the field with the crop material. The tramp metal, if it reaches the cutterhead, damages the shear bar and cutterhead knives. The problem of tramp metal in crop material being processed by agricultural harvesting equipment moving across farm fields is well known and much time and money has been expended in efforts to solve the problem.
One conventional way to alleviate the problems caused by tramp metal has been to provide a magnetic metal detector within one of the feed rolls which feeds the crop material toward the cutterhead. Metal passing in proximity to the feed roll varies a magnetic field generated by the detector and circuits are provided for detecting variations in the magnetic field and stopping the feeder mechanism when a variation is detected, thereby preventing metal objects from being fed into the cutterhead.
Many types of magnetic metal detectors have been devised in efforts to obtain a detector which is inexpensive and easily manufactured, at the same time being small enough to be located within a feed roll and sensitive enough to detect smaller metal objects such as nails, nuts and bolts anywhere over the lateral extent of the crop feed path. Various forms of prior art metal detectors are disclosed in Bennett, Jr. et al. Pat. Nos. 3,972,156, 3,757,501 and 3,889,249, Martenas Pat. No. 4,290,255, Bohman Pat. Nos. 4,433,528 and 5,070,682, Bohman et al. Pat. Nos. 5,078,645, and 5,092,818 and Strosser et al. Pat. Nos. 4,344,074 and 4,788,813. These detectors typically include a plurality of electromagnets or permanent magnets for producing a magnetic detection field and sensing coils for sensing changes in the magnetic field produced by metal objects passing through the crop feed path. Each coil may comprise a thousand turns of insulated wire. The coils are bulky and only limited space is available within the feed roll to accommodate them. Furthermore, to prevent wear of the coil insulation due to rubbing as a result of harvester vibrations, insulating pads are required between the coils and the magnets and the coils and the housing enclosing the detector.